Phase Diagrams and Tie Lines

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. Metallurgy Background 2. Lesson ObjectivesWhen you finish this lesson you will understand: Hypo and hyper euctectoid Steels How to calculate the percent of phases present in a Two phase region.. Learning ActivitiesLook up KeywordsView Slides; Read Notes, Listen to lectureView DemoDo on-li
Phase Diagrams and Tie Lines

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3. Well, we have looked at the phase transformations that occur when a steel containing exactly 0.8% carbon is cooled; let?s now look at steels containing less than that eutectoid composition. These steels are called hypoeutectoid (meaning less than eutectoid composition). Starting at the high temperature austenite phase, we slowly cool into the two phase (austenite plus ferrite) region of the phase diagram. As we enter this region, ferrite begins to form along the austenite grain boundaries. The composition of the ferrite is something just slightly less than 0.02% carbon (see the right hand end of the tie line on the diagram). This means that the austenite is very slightly increased in carbon to make up for this material which is lower in carbon. Note that at this point the volume fraction of ferrite is small so the austenite is only slightly enriched in carbon. As cooling continues in this two phase region and we approach temperature closer to the eutectoid temperature, more and more low carbon ferrite is formed, and the austenite becomes enriched in carbon up to the 0.8% carbon eutectoid composition described previously. At this temperature just above the eutectoid temperature the structure consists of grains of ferrite at 0.02% carbon decorating the austenite boundaries with the remaining austenite having composition of 0.8% carbon. When we cool to just below the eutectoid temperature, the 0.8% carbon austenite transforms to pearlite laths just as described previously, making a final microstructure of ferrite at prior austentie grain boundaries surrounding a core of pearlite structure.Well, we have looked at the phase transformations that occur when a steel containing exactly 0.8% carbon is cooled; let?s now look at steels containing less than that eutectoid composition. These steels are called hypoeutectoid (meaning less than eutectoid composition). Starting at the high temperature austenite phase, we slowly cool into the two phase (austenite plus ferrite) region of the phase diagram. As we enter this region, ferrite begins to form along the austenite grain boundaries. The composition of the ferrite is something just slightly less than 0.02% carbon (see the right hand end of the tie line on the diagram). This means that the austenite is very slightly increased in carbon to make up for this material which is lower in carbon. Note that at this point the volume fraction of ferrite is small so the austenite is only slightly enriched in carbon. As cooling continues in this two phase region and we approach temperature closer to the eutectoid temperature, more and more low carbon ferrite is formed, and the austenite becomes enriched in carbon up to the 0.8% carbon eutectoid composition described previously. At this temperature just above the eutectoid temperature the structure consists of grains of ferrite at 0.02% carbon decorating the austenite boundaries with the remaining austenite having composition of 0.8% carbon. When we cool to just below the eutectoid temperature, the 0.8% carbon austenite transforms to pearlite laths just as described previously, making a final microstructure of ferrite at prior austentie grain boundaries surrounding a core of pearlite structure.

4. This just provides a closer look at the hypoeutectoid steel phase transformation upon equilibrium slow cooling.This just provides a closer look at the hypoeutectoid steel phase transformation upon equilibrium slow cooling.

5. And here is provided an actual photomicrograph of this hypoeutectoid steel slowly cooled microstructure. By looking at the volume fraction of ferrite and peralite, can you guess at what the original (nominal) composition of the austenite might be?And here is provided an actual photomicrograph of this hypoeutectoid steel slowly cooled microstructure. By looking at the volume fraction of ferrite and peralite, can you guess at what the original (nominal) composition of the austenite might be?

6. Another photomicrograph of a hypoeutectoid steel with significanly more ferrite (that is lower nominal carbon content in the austenite). Can you guess what the carbon content of this material might have been?Another photomicrograph of a hypoeutectoid steel with significanly more ferrite (that is lower nominal carbon content in the austenite). Can you guess what the carbon content of this material might have been?

7. A tie line in the two phase region of a phase diagram is used to determine how much of each phase might be present and the composition of those phases.A tie line in the two phase region of a phase diagram is used to determine how much of each phase might be present and the composition of those phases.

9. A tie line in the two phase region of a phase diagram is used to determine how much of each phase might be present and the composition of those phases. Doing this assignment will give you the concept of tie lines and practice doing tie line calculations. Please review this demonstration and do this homework as this is a most important concept. If you have difficulty with this, please ask for help in class or during office hours.A tie line in the two phase region of a phase diagram is used to determine how much of each phase might be present and the composition of those phases. Doing this assignment will give you the concept of tie lines and practice doing tie line calculations. Please review this demonstration and do this homework as this is a most important concept. If you have difficulty with this, please ask for help in class or during office hours.


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